1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to an image forming apparatus such as a lazar printer or a digital copying machine which carries out modulation control of the output light of a semiconductor laser so as to use as an optical writing apparatus.
2. Description of Related Art
The semiconductor laser is small, and is widely used as a light source of an optical writing apparatus used for an image forming apparatus such as a laser printer or a digital copying machine since high-speed modulation can be performed. However, since the relationship between the drive current of a semiconductor laser and an optical output changes with temperature remarkably, a problem may occur when an attempt is made to set an optical intensity of a semiconductor laser to a desired value.
APC (Automatic Power Control) system is known as a method of using the semiconductor laser, which solves such a problem. In APC system, an optical output of a semiconductor laser is monitored by a light-receiving element, and a normal direction current of the semiconductor laser is always controlled to be a desired value by an optical-electronic negative feedback loop which controls the normal direction current of the semiconductor laser so that a signal proportional to a light-receiving current proportional to the optical output of the semiconductor laser generated in the light-receiving element and a luminescence level instruction signal become equal to each other.
Japanese Laid-Open Patent Applications No. 05-075199, No. 05-235446, No. 09-321376, No. 11-167081, and No. 05-207234 discloses a technology with respect to the above-mentioned APC system.
Japanese Laid-Open Patent Application No. 05-75199 discloses a semiconductor laser control apparatus which achieves high-speed modulation while controlling a temperature characteristic and a droop characteristic of a semiconductor laser by constituting an optical-electric negative feedback loop for controlling the semiconductor laser by continuously comparing a light-receiving current and a luminescence instruction current of a light-receiving element which monitors an optical output of a semiconductor laser, and by modulating the semiconductor laser at a high speed by supplying to the semiconductor laser an output current of the optical-electric negative feedback loop by adding a current proportional to the luminescence instruction current.
Japanese Laid-Open Patent Application No. 05-235446 discloses a semiconductor laser control apparatus which incorporates a semiconductor laser protection circuit with a simple structure for preventing a surge current when a power is turned on and degradation of the semiconductor laser by an excessive current in an unstable state of circuits.
Japanese Laid-Open Patent Application No. 09-321376 discloses a technology which is realized so that a semiconductor laser control apparatus applied to an image forming apparatus, to which a current adding system that reduces an amount of control by an optical-electric negative feedback loop and a pulse width intensity mixing modulation system in a single dot are applied, is realized with smaller size and power saving and further higher integration.
Namely, Japanese Laid-Open Patent Applications No. 5-075199, No. 5-235446 and No. 9-321376 suggest realization of a control of a temperature characteristic and droop characteristic and high-speed modulation by the method of constituting an optical-electric negative feedback loop for controlling the semiconductor laser at a high speed by continuously comparing a light-receiving current and a luminescence instruction current of a light-receiving element which monitors an optical output of a semiconductor laser, and by modulating the semiconductor laser at a high speed by supplying to the semiconductor laser an output current of the optical-electric negative feedback loop by adding a current proportional to the luminescence instruction current.
However, due to the characteristic of the light-receiving element that monitors the optical output of a semiconductor laser, the linearity of the light-receiving current output characteristic with respect to the optical input of the light-receiving element will deteriorate remarkably if the optical output of a semiconductor laser becomes small. For this reason, the control accuracy in the case of a low light output may become bad, and there may be a case in which an optical output is larger than a predetermined optical output. In such a case, there is a possibility of having bad influences, such as background contamination, in a laser printer and the like.
Moreover, since the optical output is always controlled, an optical output cannot be turned off completely so as to have the control system to carry out a normal operation, thereby making an offset light generated. Moreover, a drive current setting circuit, which sets up the drive current to the semiconductor laser, is needed, and restrictions may be given to a circuit scale when an attempt is made to improve the function of optical modulation ICs of a laser printer or the like.
Furthermore, since the light-receiving element which detects only optical output of a single semiconductor laser is needed, means is required externally for separating and detecting optical outputs to separate and detect each optical output when one light-receiving element detects the output of a plurality of lasers such as a semiconductor laser array.
Moreover, Japanese Laid-Open Patent Application No. 11-167081 discloses a pixel clock frequency setting method by a direct synthesizer. According to this method, a frequency change can be performed at a high speed by changing a frequency unit by changing data of LUT (look-up table). However, since a frequency variable unit and output frequency change speed are closely related to the control speed of a subsequently connected PLL-LOOP and low-pass passage filter, restrictions are given at the time of designing the entire structure. Moreover, a frequency unit is dependent on a master clock frequency and the number of bits of LUT, and in order to perform a fine setup, it is required to increase a circuit scale or to make a master clock a high speed, thereby accompanying difficulties to realize a single chip structure.
Moreover, although Japanese Laid-Open Patent Application No. 5-207234 discloses a method of adding a phase error to PLL-LOOP, a frequency error of a pixel clock is generated in this method unless an addition signal of phase error stability is very stable. This will become large restrictions in an attempt to make a single chip IC by unifying a digital circuit and an analog circuit.
Next, a further description will be given, with reference to FIG. 1, of a conventional image forming apparatus. In FIG. 1, a laser light emitted from a semiconductor laser unit 21 is scanned by rotation of a polygon mirror 22. The scanned laser light forms optical spots on a scanned medium (photo conductor) 24 through a scanning lens 23, exposes the scanned medium 24 and forms an electrostatic latent image. At this time, formation of the electrostatic latent image on the scanned medium 24 is controlled by controlling the light-emitting time of the semiconductor laser based on image data generated by the image-processing unit 26 and an image clock of which phase is set up by a phase synchronous circuit 29. Moreover, the phase synchronous circuit 29 sets the phase of the clock generated by a clock generating circuit 28 to a phase which is synchronized with a photodetector which detects the light of the semiconductor laser which is scanned by the polygon mirror 22.
As mentioned above, in the image forming apparatus which uses a laser scanning optical system, a laser drive circuit 27, the phase synchronous circuit 29 and the clock generation circuit 28 are indispensable in a position accuracy and interval accuracy of the electrostatic latent image formed on the scanned medium 24. For this reason, a clock having the same frequency with the image clock is needed for many circuits in the image forming apparatus, and there is a possibility of causing a problem of EMI (Electro Magnetic Inference) of the image forming apparatus. Moreover, it also induces a cost rise due to an increase in the number of parts. Furthermore, it becomes very difficult to operate an image data transfer clock at completely the same timing in entire system as a printing speed increases, and, thereby, data must be transferred in parallel by image data transfer with a slow clock.
Moreover, in recent years, a multi-beam optical system which attains high speed and high density is being adopted in connection with the high speed and high density of a laser printer by recording not only by a light from a single light source but by lights from a plurality of light sources. However, there are a case in which a plurality of semiconductor lasers are used as a light source and a case in which a semiconductor laser array on which a plurality of light-emitting points are arranged in monolithic on a single chip, and it is preferable that these are selected, if necessary, from a systematic viewpoint.
However, conventionally, since the light-receiving element is common to all semiconductor lasers with respect to the semiconductor laser array, the methods recited in the above-mentioned Japanese Laid-Open Patent Applications No. 5-75199, No. 5-235446, No. 9-321376, etc. cannot be used, which results in an increased cost when the semiconductor laser array is used.
Moreover, although a continuous control is needed in order to remove the influence of the temperature characteristic, the droop characteristic, etc. of the semiconductor laser as recited in Japanese Laid-Open Patent Applications No. 5-75199, No. 5-235446, No. 9-321376, etc., an offset light may arise so as to perform a simultaneous, continuous control. Moreover, a current setting circuit, etc. is needed which results in an increase in a circuit scale. Furthermore, when a semiconductor laser array is used, means for separating and detecting each optical output must be externally provided.
Moreover, the beam profile of a semiconductor laser is usually approximated to Gaussian distribution, and an electrostatic latent image in an electronic photograph system is formed according to Gaussian distribution. For this reason, the electrostatic latent image is not in binary values, and analog-distribution parts generate according to an increase in resolution. Accordingly, it is easily influenced by external fluctuation factors, such as changes in a development bias, and it tends to easily cause fluctuation in image intensity.
Furthermore, although the pixel clock frequency setting method by the direct synthesizer which is disclosed in Japanese laid-Open Patent Application No. 11-167081 can change frequency at a higher speed by changing the frequency unit by changing the data of LUT, a frequency variable unit and output frequency change speed closely relate to control speed of PLL-LOOP and the low-pass filter, which are subsequently connected, and give restrictions on the entire structure design. Moreover, since a frequency unit is dependent on a master clock frequency and the number of bits of LUT, in order to perform a fine setup, it will need to increase a circuit scale or increase a master clock speed, which accompanies difficulty in realizing a single chip IC.
Moreover, in the method of adding a phase error to PLL-LOOP as disclosed in Japanese Laid-Open Patent Application No. 5-207234, the frequency error of a pixel clock may occur unless the addition signal of the phase error is stable very much. This may give large restrictions when attaining a single ship IC by unifying a digital circuit and an analog circuit.
Moreover, in a deflector such as a polygon scanner, the variation in the distance from the axis of rotation of a deflection reflective surface (variation in an inscribed circle radius) generates unevenness in the scanning speed of the optical spot (scanning beam) which scans a surface to be scanned. After detecting synchronous light, a write-in signal is issued at a predetermined timing and a semiconductor laser starts light emission, data corresponding to a single scanning line is sent to each source of light emission, and thereby, an image is formed as a latent image on a scanned medium.
At this time, as shown in FIGS. 2A and 2B, unevenness (variation) of the scanning length of each scanning line appears according to the above-mentioned factor in the deflector such as a polygon scanner. Such unevenness is mainly conspicuous at an edge of image as well as a write-in magnification error, and the variation in the above-mentioned write-in end appears as fluctuation of an image end. The influence on the image by the factor of the above-mentioned deflector is larger as closer to the end, and degradation of image quality is conspicuous, although fluctuation of an image is also generated not only at an end but also in the middle. This degradation of the image quality by fluctuation of an end needs to be corrected when there is a demand for a high-definition quality of image.
Furthermore, in the case of a multi-beam optical system, it is necessary to correct scanning width since when there is a difference in the oscillation wavelength of each source of light emission, an offset of exposure position occurs in a case of an optical system in which a color aberration of a scanning lens is not corrected, and a difference is generated in the scanning with, when an optical spot corresponding to each source of light emission scans on the scanned medium, for each source of light emission, which becomes a cause of degradation of image quality.
Moreover, the interval of light-emitting points of a semiconductor laser array has a limit in positioning the light-emitting points closer to each other due to influences of a thermal cross-talk or an electric cross-talk. Moreover, it becomes a cost demerit to make many kinds of light-emitting point intervals of a semiconductor laser array. However, various scanning optical systems have been developed with the write-in density or scanning width, and there are various magnifications of a scanning optical system. Therefore, in order to obtain arbitrary scanning pitches on a surface to be scanned, the pitch of the light-emitting point is apparently made into a desired pitch in the sub-scanning direction by slanting a semiconductor laser array. However, when a semiconductor laser array is slanted, the scanning start position on the surface to be scanned which is formed by a light flux projected by each light-emitting point is shifted. Moreover, even when the semiconductor laser array is not slanted, the scanning start position on the surface to be scanned shifts due to position offset of the light-emitting point caused by a machining error during a manufacturing process of the semiconductor laser array. Since this becomes a cause of degradation of image quality, it needs to correct the scanning start position.
Furthermore, in the case in which a light source part of a multi-beam optical system is constituted by combining a plurality of semiconductor lasers, it is necessary to correct a scanning start position since there is a problem that the scanning start position shifts in the same manner as mentioned above, which becomes a cause of degradation of image quality.
Moreover, in an optical system design, although a high performance (reduction of an image surface curve, reduction of a magnification error, reduction of a scanning line bend, etc.) of an optical system is attained for achieving a high image quality of an output image, there is a limit also in it due to restrictions from the number of sheets, field composition and the quality of the material of the optical element of an optical system. Therefore, in order to attain further high performance, the increase in the number of sheets of the optical elements, introduction of a special form surface and use of an expensive optical material are needed, and, thus, issues related to a cost increase, an improvement in design difficulty and an improvement in machining difficulty of the optical system arises.